Acute respiratory tract infections are a leading cause of morbidity and mortality in children worldwide. Human metapneumovirus (hMPV) is a recently identified human pathogen responsible for a significant portion of upper and lower respiratory tract infections not only in children but also in the elderly and in immunocompromised patients. No effective treatment or vaccine for hMPV is currently available and many fundamental questions regarding the pathogenesis of hMPV-induced lung disease and the host immune response have yet to be answered. We have recently found that hMPV glycoprotein G expression potently inhibits type I interferon (IFN) production, as well as secretion of cytokines and chemokine both in vitro and in vivo. This effect occurs via inhibition of viral-induced Nuclear Factor-?B (NF-?B) and Interferon Regulatory Factors (IRF) activation, suggesting a role of G protein in regulating early intracellular signaling events triggered by hMPV infection. Indeed, we found that G protein specifically targets hMPV-induced cellular responses mediated by the cytoplasmic RNA helicase retinoic acid-inducible gene-I (RIG-I) in airway epithelial cells and by Toll-like receptor (TLR)4 in primary immune cells. In this grant, we propose to identify the mechanism(s) by which hMPV G protein inhibits TLR-dependent and -independent cellular signaling and to start defining the role of G protein in modulating innate and adaptive immune responses in vivo. Upon completion of the proposed investigations, we will obtain new critical information regarding the mechanisms of hMPV-induced cellular signaling, which may allow us to specifically modulate viral-induced gene expression and therefore antiviral and innate immune/inflammatory responses. Furthermore, the results obtained from these studies will be instrumental for the development of safer and more effective hMPV vaccines. PUBLIC HEALTH RELEVANCE: Human metapneumovirus (hMPV), a recently identified virus, is a major cause of bronchiolitis, pneumonia and flu-like syndromes, as well as asthma exacerbations, and it is now considered a substantial public health problem for the community. We have recently identified hMPV glycoprotein G as an important virulence factor, responsible for inhibiting innate immune responses to hMPV infection. The aim of this grant application is to investigate the role of hMPV glycoprotein G in modulating host cellular responses using a combination of molecular virology, cellular biology and immunology techniques in order to develop novel therapeutic strategies and safer and more effective vaccine candidates to reduce the morbidity and mortality associated with hMPV infections.